Horsepower Calculator

About the Horsepower Calculator

A horsepower calculator converts between torque and RPM to compute engine power output, and converts between the different horsepower standards — mechanical (imperial), metric (PS/CV), and electrical. Horsepower is the most common unit of power for vehicle engines in the USA, UK, and Canada, while European specifications often use metric PS (Pferdestärke/cheval-vapeur). One mechanical horsepower equals 745.7 watts; one metric PS equals 735.5 watts — a difference of about 1.4%. The core relationship is HP = Torque (lb-ft) × RPM / 5252, where 5252 is a constant derived from converting RPM to radians per second. Dyno sheets plot both torque and horsepower curves against RPM; they always cross at exactly 5,252 RPM because of this formula. This calculator is used by automotive enthusiasts comparing engine specs, mechanics interpreting dyno results, students studying engine thermodynamics, and engineers converting between power standards.

How It Works

Power (watts) = Torque (N·m) × Angular velocity (rad/s). In imperial units: HP = Torque (lb-ft) × RPM × 2π / (60 × 550). Simplifying: HP = Torque (lb-ft) × RPM / 5252. The constant 5252 = (60 × 550) / (2π) ≈ 5252.1. Conversions: 1 mechanical HP = 745.7 W = 0.7457 kW. 1 metric HP (PS) = 735.5 W = 0.7355 kW. 1 electrical HP = 746 W exactly. Example: engine produces 300 lb-ft of torque at 4,000 RPM. HP = 300 × 4000 / 5252 = 228.5 HP. At 5252 RPM: if still 300 lb-ft torque, HP = 300 × 5252 / 5252 = 300 HP — torque and HP are equal. Above 5252 RPM, HP exceeds torque in lb-ft figures.

Tips & Best Practices

• ✓Horsepower and torque are mathematically related — you cannot have one without the other. High-torque engines (diesels, large V8s) produce peak torque at low RPM; high-revving engines (sports cars, motorcycles) produce peak HP at high RPM. Neither metric alone tells the full story of performance.
• ✓Peak HP and peak torque occur at different RPM. A typical naturally aspirated petrol engine peaks torque at 3,000-5,000 RPM and peak HP at 5,000-7,000 RPM. Turbocharged engines peak torque much lower (1,500-3,000 RPM) — this is why turbocharged cars feel fast from low speeds.
• ✓For electric motors: rated power is typically continuous, not peak. A motor rated at 150 kW (201 HP) can sustain that output; a comparable combustion engine rated at 150 kW might only sustain its peak power for a few seconds. Electric motors also produce maximum torque from 0 RPM, while combustion engines build torque as RPM increases.

Who Uses This Calculator

Automotive enthusiasts comparing engine specifications and dyno results between vehicles. Mechanics and tuners calculating expected power from measured torque values. Students in automotive engineering programmes learning power and torque relationships. Vehicle buyers comparing US HP ratings with European PS specifications to ensure fair comparison.